The invention relates to a device for controlling electric current flowing through a condensation-free transparent glass sheet to eliminate vapor condensation thereon by means of electric heating elements.
Condensation formed on the glass window of an automobile obscures the driver's view and hampers driving. To solve this problem, it has been the practice to coat the glass window with a transparent conductive film or a plurality of conductive lines or stripes which serve as heating elements. The same measures are also taken for a similar problem experienced with refrigerators. The on-off control of the power supply for conductor-coated glass sheets is usually performed by manual switches or automatic timer switches. In the former, however, the switching operation is troublesome and tends to involve excessive power consumption due to neglect by the operator. Likewise, in the latter, it is hard to set the optimum operating period of the heating elements. Therefore, a considerable amount of power loss will result. There is another system in which a thermistor is attached to the surface of the glass sheet, so as to control the heater current in accordance with the detected temperature, and thereby maintain the glass sheet at a constant temperature. It is well known, however, that the dew point depends not only on ambient temperature but also on humidity. This gives rise to difficulties in achieving the optimum temperature setting for the thermistor. For these reasons, excessive power consumption necessarily results even in the thermistor -- controlled system.
A pair of moisture-detecting electrodes attached to the surface of the glass sheet may be employed to turn the heater current on and off in response to variations in resistance caused by condensation. Thus, the heating can be controlled in accordance with the moisture or the degree of condensation on the surface of the glass sheet. Power consumption is minimized in this system. However, if a DC voltage is applied across the dewdetection electrodes, they tend to become corroded due to the ion transfer caused by the applied potential. This problem may be avoided by the use of corrosion-resistant materials of precious metals, e.g. platinum (Pt), gold (Au), palladium (Pd). These materials are costly, however, and differ from the materials ordinarily used for the heating elements, for example, a glass film containing silver powder baked to form stripes on the surface of the glass sheet or a transparent coating of tin oxide. Accordingly, separate steps are required for manufacture where the detecting electrodes are corrosion-resistant. The production process is simplified if the detecting electrodes and heating elements are made of the same materials.